1. Field of the Invention
The present invention relates to a mask pattern data creation method and a mask used to form, for example, a semiconductor integrated circuit.
2. Background Art
It is known that during the exposure and transfer of relatively densely disposed periodic patterns such as, in particular, patterns for the formation of contact holes, process margins (an exposure amount margin and/or a focus margin) are improved in comparison with a perpendicular illumination by using a modified illumination such as a dipole illumination or a quadrupole illumination that irradiates obliquely onto a photomask. Therefore, assist pattern features referred to as SRAFs (sub-resolution assist features), etc., are formed on the mask in a size below the resolution limit during the wafer transfer so as not to transfer onto the wafer, and are disposed adjacent to actual patterns (main pattern features) to be transferred onto the wafer. This technology is known to improve the margin by artificially providing a periodicity to the pattern entirety (for example, JP-A 2008-66586 (Kokai)).
However, in the case where the main pattern features (the actual patterns to be transferred) are disposed at random pitches, adding assist pattern features of the same design uniformly to all main pattern features may result in cases where the maximum margin improvement effects due to the assist pattern features are not obtained for some pitches.